Arthritis is one of the most common causes of functional limitation and dependency in the United States. Individuals with osteoarthritis restrict joint motion and limit activity in order to decrease their symptoms. Traditional, conservative medical treatment of osteoarthritis has been directed at improving functional status through reducing joint pain and inflammation and maintaining or restoring joint function. Exercise as an adjunct therapy in the clinical management of patients with osteoarthritis of the knee, however, is not uniformly accepted. In contrast, exercise has been shown to be effective for prevention and treatment of cardiovascular and metabolic disorders. Standard guidelines exist for aerobic exercise prescriptions. The focus of this study is to determine if these guidelines can also be applied to individuals with knee osteoarthritis. Patients with knee osteoarthritis will be randomized into a control group, a walking exercise group, and a stationary cycling exercise group. The individuals in the exercise groups will be required to exercise three times per week for one year using emerging public health recommendations for aerobic exercise in the adult and aging population. Patient outcome will be assessed using objective gait analysis measurements, knee radiographs to quantify joint space narrowing, magnetic resonance imaging, a general health status questionnaire (SF-36), a disease/site specific questionnaire (WOMAC), and a visual-analog pain scale. All subjects will be studied at 0 and 52 weeks. The central hypothesis of this work is that aerobic exercise can be successfully implemented as an effective nonsurgical option for treatment of patients with early stages of knee osteoarthritis. In order to evaluate this hypothesis, the following specific aims are proposed: Specific Aim 1: Determine the effect of aerobic exercise on patients with knee osteoarthritis. Hypothesis A: Clinical Outcome measures will be better in patients enrolled in exercise programs that in control patients. Hypothesis B: Quantitative measures of lower extremity function will not decline over time in an effective aerobic exercise program. Specific Aim 2: Determine prognostic factors that effect outcome in patients with knee osteoarthritis. Hypothesis: A effective exercise prescription for adults with degenerative joint disease is dependent on knee compartment involvement, CIA stage, BMI, and type of exercise prescribed.